The present disclosure relates to a carrier and a two-component developer.
In general, in electrophotography, the surface of a photoconductor drum is charged by a method such as corona discharge, followed by exposure using a laser etc. to form an electrostatic latent image. The formed latent image is developed with a toner so as to from a toner image. The formed toner image is transferred onto a recording medium to obtain an image with high quality. The toner used for formation of a toner image is typically a toner including toner particles (toner base particles) with an average particle diameter of 5 μm or larger and 10 μm or smaller produced by mixing a binder resin such as thermoplastic resin with components such as a colorant, a charge control agent and a release agent, followed by a kneading step, a pulverization step and a classification step. For the purpose of providing flowability or suitable charging performance for the toner particles, and/or for facilitating cleaning of the toner particles from the surface of the photoconductor drum, silica and/or inorganic fine particles such as those of titanium oxide are externally added to the toner base particles.
As such a developing system using such a toner, a one-component developing system using the toner independently as a developer (one-component developer), and a two-component developing system using a developer formed by mixing toner and carrier (two-component developer) are known. Then, in the two-component developer system using a two-component developer, the carrier particles cause the toner particles to be charged by frictional electrification, as well as bearing the role of transporting toner particles. For this reason, there is an advantage in that the electrostatic property and transport property of the toner particles when beginning image formation tend to be comparatively stable.
Conventionally, as the carrier used in such a two-component developing system, a magnetic carrier composed of particles of a metal with large specific gravity like magnetite and ferrite have been used. However, when using such metallic particles as the carrier, the mechanical load acting on the stirring unit mixing the two-component developer inside a developing unit positioned in the image formation apparatus may become excessively large.
For this reason, in order to decrease the mechanical load acting on the stirring unit upon forming an image using a two-component developer, a magnetic dispersion-type resin carrier in which magnetic material fine particles are dispersed in a binder resin of low specific gravity has been proposed. The magnetic material dispersion-type resin carrier has a small specific gravity compared to a carrier composed of particles of metal; therefore, the load acting on the stirring unit upon forming an image using the two-component developer is mitigated. It is thereby possible to reduce the size of the motor driving the stirring unit when using a two-component developer containing a magnetic material dispersion-type resin carrier, and thus energy savings and a size reduction are achieved in the image formation apparatus based on a two-component developing system.
However, when repeatedly forming images using a two-component developer containing a magnetic material dispersion-type resin carrier, the magnetic material may drop out from the carrier particles from the repeated stress acting on the carrier particles. If the magnetic material drops out from the carrier particles, the ability of the carrier particles to charge the toner particles will decline, and oppositely charged toner particles may tend to generate, which leads to scattering of toner particles. For this reason, an improvement in the durability of the carrier has been desired.
Therefore, with the object of improving the durability of the carrier, a magnetic material dispersion-type resin carrier has been proposed that is composed of ferromagnetic iron oxide fine particles and cured phenol resin, and has a cover layer composed of melamine resin formed on the particle surface of magnetic material dispersion-type core particle.
However, for the above-mentioned magnetic material dispersion-type resin carrier, the phenol resin contained in the core particles and the melamine resin constituting the cover layer of the core particle surface generally have low affinity. For this reason, in the case of forming images for a long time using the two-component developer containing the proposed magnetic material dispersion-type resin carrier, separation of the coating resin will occur. Then, if separation of the coating resin occurs, there is a possibility of dropping out of the magnetic material occurring, similarly to the conventionally known magnetic material dispersion-type resin carrier. For this reason, further improvement in the durability of the carrier has been demanded.